Eutrophication of Marine Waters: Effects on Benthic Microbial Communities

During the last century organic pollution in coastal areas of the sea has become a serious world problem. One of the major stresses comes from the input of excessive macronutrients (nitrogen, phosphorus) resulting in a change of the trophic status of a given body of water, which leads to eutrophication. Although the effects of eutrophication are well-known, the mechanisms governing its effects are poorly understood. In particular, effects on microbial processes are key to many aspects of the functioning of the ecosystem, and commonly are inadequately addressed. The effects of eutrophication on benthic microbial communities are demonstrated using shallow-water coastal inlets in the southern Baltic Sea as an example. These so-called ‘Boddenʹ are characterized by pronounced gradients of inorganic and organic nutrients. For the hypertrophic innermost parts of the Bodden, critical points can be identified at which the chronic stress caused by eutrophication could no longer be compensated for by the system. Signs of eutrophication of sediments of the Bodden include increases in inorganic and organic carbon, nitrogen and phosphorus, microbial biomass and enzymatic decomposition potential of substrates, nitrification, denitrification, and nutrient fluxes from the sediments, all of which can be measured. Above certain carbon concentrations, further increases in organic carbon are not necessarily paralleled by corresponding increases in biological parameters. This might be taken as an indication of a different status of nutrient enrichment. Eutrophication effects became most obvious from changes in the ratios of pelagic to benthic primary production, oxygen to sulphate respiration, and proteolytic to carbohydrate decomposing enzyme activities. The structure and function of microbial biofilms colonizing stones and sediments also reflected the changed trophic status. With increasing eutrophication, the ratio of autotrophic to heterotrophic microbial processes becomes greatly reduced. Drifting filamentous macroalgae, mats of sulphur oxidizing and anaerobic phototrophic bacteria, represent visible signs of eutrophication. Although the external nutrient loads in the example of the Bodden have been greatly reduced during the last decade, the internal loads of the sediments remain a serious problem. Remediation concepts can only support the natural self-purification potential of a marine coastal ecosystem.